The present invention relates to a parameter estimation method for a self-energized brake mechanism, and a device for executing such a method.
With conventional disc brake mechanisms the frictional brake force has no feedback to the actuator force or the clamping force. This means that the brake mechanism is not capable of determining any relation between the clamping force and the friction force. However, in case of self-amplified wedge brakes, where one part of the actuation energy is generated from the friction force, the friction coefficient can be reconstructed because the friction force has a direct feedback to the actuator movement.
In conventional commercial vehicle brake systems, the control feedback is the pressure which has a good relation to the clamping force. In this case, the value of an air gap between a brake pad and a brake disc influences the relation only marginally, so it is taken into consideration via threshold pressure. In self-amplifying brake mechanisms, the air gap between the brake pad and the brake disc has a more significant influence so its value is also useful information for a brake controller.
With self energized wedge brakes, considering the cost optimal realization, the friction force cannot be measured effectively in direct ways, moreover, the clamping force, being the analogous signal to the brake pressure of current brakes, is also not always measured by sensors. However, the friction coefficient is one of the most important parameters (a disturbance signal) of the system since it determines the magnitude of the self-amplification as well as being a further factor between the clamping force and the friction force. Thus the friction coefficient needs to be reconstructed from the other measured signals.
In the case of the electro-mechanical brake systems, the actuator position can be easily measured (e.g. as a motor position), which has a certain correlation with the clamping force. However, this correlation contains a new uncertainty, namely the air gap between a brake pad and a brake disc. This disturbance signal should also be reconstructed in order to adjust low brake force levels accurately if there is no direct clamping force measurement.
WO 2007/073927 A1 describes a method for determining the friction value of a disc brake, in particular of self-energizing disc brakes. The determination of the friction value is carried out mathematically on the basis of the following variables: motor current for the application and retraction directions; idle current; wedge angle; transmission constant and clamping force.
WO 2003/036121 A1 shows a self-energizing electromechanical disc brake comprising a rotatable brake disc and an electric actuator that generates an actuating force acting on a friction lining via a wedge system against the brake disc. Also provided is a device for detecting the moment of friction, which comprises first means for measuring the friction force and second means for detecting the force perpendicular to the brake disc, or first means for detecting the force of the actuator and second means for detecting the force perpendicular to the brake disc.
These solutions for friction coefficient estimation calculate the estimate based on static relations and on the assumption that the static load of the actuator can be separated from the dynamic load in practice as well.
Methods for estimating a torque/force exerted by a load against an actuator driven by an electric motor against the load are described in US 2007/0085414 A1. A first method includes measuring motor current of the electric motor and measuring a position/angle or speed/angular speed of the actuator and includes calculating the torque/force exerted by the load against the actuator using at least a difference between a calculated motor torque/force and a calculated actuator-experienced torque/force. The effect of the load is a “Disturbance Torque” calculated as an “Observed Disturbance Torque” and can be mathematically converted and then used as an “Observed Disturbance Force” by a controller to compute an “Input Voltage” to the electric motor to control the brake. A second method includes measuring input voltage of the electric motor instead of measuring motor current, and a third method includes measuring both motor current and input voltage. The actuator is an automotive electromechanical brake caliper. The methods use mathematical models of the actuator. The mathematical models are mathematical models of the mechanical aspects of the electromechanical actuator. Estimating the actuator load can be a disadvantage due to time consuming calculation time.
In view of the above, it is an object of the present invention to provide an improved parameter estimation method for a self-energized brake mechanism. Another object of the present invention is to provide an improved device for executing such a method.
According to the present invention, this object is achieved with a method for estimating parameters for a self-energized brake mechanism having a moving part, a brake pad, a brake disc, a caliper, and a wedge profile, the method including the following steps: applying the same actuator force to the brake mechanism and simultaneously to a dynamical model of the brake mechanism; calculating deviations based on measured state variables of the brake mechanism and simulated state variables of the dynamical model; and producing the estimated parameters by online minimization.
According to the present invention, this object is also achieved with a device for parameter estimation for a self-energized brake mechanism having a moving part, a brake pad, a brake disc, a caliper, and a wedge profile, the device including: a brake mechanism with measuring device for state variables; a dynamical model for calculating simulated state variables; a deviation unit for comparing the measured state variables and the simulated state variables; and a minimization unit for providing estimated parameter values, wherein inputs of the brake mechanism and the dynamical model are connected in parallel and outputs of the minimization unit are connected with the dynamical model for feedback.
The estimation of the parameters of a self-amplifying brake system uses the properties of the self-energized brake system and does not contain any intermediate step for estimating the actuator load. Therefore, the method of the present invention is capable of covering any stationary or transient situation and saves calculation time.
The method is based on the dynamical model of the system. The same actuator force input to the dynamical model as to the real brake mechanism is applied simultaneously. Therefore, only one input value is needed.
At least the position of the moving part is measured, and optionally the speed thereof; otherwise the actuator speed is reconstructed from the measured position. A calculation of deviation norms based on the measured and simulated state variables is executed.
The parameters of the model, to which the system is sensitive like friction coefficient and air gap, are adjusted, which provides the main output of the estimated parameters in order to minimize the deviation norm. The estimated parameters are fed back to the dynamical model in order to obtain an optimized output.
It is a parameter identification method which can be executed in the brake controller online in real-time. Furthermore, the method can be applied for non self-amplifying brakes as well as for estimation of the air gap only.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.